Magnesium control in manganese electrowinning



May 16, 1944. I 'c MANTELL ET AL 2,348,742

MAGNESIUM CONTROL IN MANGANESE ELECTROWINNING Filed Oct. 25, 1940Sheets-Sheet l Mal W/ am/ f/axhmer ca'af- INVENTOR5 Patented May 16,1944 MAGNESIUM EONTROL IN MANGANESE ELECTRQWWNING Charles L. Mantell,Manhasset, N. Y and William L. Hammerqulst, Knoxville, Tenn, assignorsto Electro Manganese Corporation, Minneapolis, Minn, a corporation ofDelaware Application il ctoher 25, 1940, Serial No. 362,802

(Cl. 2M-l05) 8- Claims.

This invention relates to improvements in the commercial electrowinningof manganese from its ores.

In the commercial electrowinning of manganese a body of electrolyte ismaintained in an electrolytic cell provided with an anode and a cathode,separated by a permeable membrane or diaphragm into anolyte andcatholyte chambers, respectively. From this body of liquid, specificallyfrom the catholyte in the catholyte chamber, manganese is removed in theform of me-' tallic manganese and deposited on the'cathode. To this bodyof liquid a manganese salt is added to replenish the solution withmanganese ions and compensate for the removal of metallic manganese. Themanganese salt is obtained by leaching a suitably conditioned manganeseore with an acid solution and in practice this acid solution isthe-anolyte from the anolyte chamber. Nothing is removed from theelectrolyte except metallic manganese and the volatile products ofelectrolysis whereas there is a continuous or progressive addition tothe electrolyte of the products obtained by leaching ore with theanolyte liquid.

Consequently the concentration of impurities, specifically metallicelements other than manganese, continuously increases unless steps aretaken to prevent this increase in concentration of impurities. Amongthese impurities magnesium is a common constituent of manganese ores. Toillustrate this, reference may be had to the accompanying drawings inwhich Fig. 1 shows diagrammatically acell for the electrowinning ofmanganese and its associated leaching and purification system. The celli is divided into an anolyte compartment 2 and a catholyte compartment 3by the permeable diaphragm 4. An electric current may be passed throughthe cell between the anode 5 and cathode 6. The permeable diaphragm maybe made of textile material or other permeable substance. There is atransfer of catholyte to anolyte by diffusion through this diaphragm andan overflow pipe 1 permits flow of catholyte from the catholyte to theanolyte chamber. In practice the cell contains a series of anodes andcathodes arranged alternately in a row and separated by a correspondingseries of diaphragms. The catholyte and anolyte compartments arerespectively connected together bysuitable manifolds. The cathodes andanodes are also respectively electrically connected. Fig. 1, however,shows a single catholyte and single anolyte chamber separated by adiaphragm and provided with electrodes as shown, for the sake ofsimplicity since the particular construction of the cell is not a partof the present invention. Line 8 con- "ducts anolyte from the anolytechamber to a ments is difierent.

tank a in which-suitably conditioned manganese ore is extracted. Afterthe removal of insoluble material the liquid extract is then subjectedto a series of operations to remove impurities including iron, nickel,cobalt and arsenic, the tank l0 being designateddiagrammatically toindicate apparatus in which this series of operations may be performed.

Owing to the different electrolytic changes occurring in catholyte andanolyte the composition of the liquids in these respective compart- Forexample, the composition of the anolyte may be represented as follows:pH 0.5 to 1.5 represented by sulfuric acid in the case where manganeseis used as manganese sulfate, ammonium sulfate to grams per liter,manganese (for example as manganese sulfate) 5 to 15 grams per liter, apreferred range being 13 to 15 grams per liter.

When this solution is used to extract conditioned manganese ore in thetank 9 the basic character of the ore changes the pH and the liquidremoved from the extract tank Q may have a pH of about 2.0 to 4.0.Simultaneously, of course, the concentration of manganese is increasedto a value which may be represented by 25 to 55 grams of manganese perliter, a preferred range being 35 to 55 grams per liter, in the form ofa soluble manganese salt, as for example the sulfate. In the extractiontank ll the anolyte liquor becomes contaminated with impurities, as forexample iron, cobalt, nickel,

arsenic, calcium and magnesium. The extent of this contamination iscontrolled by the solubility of the respective sulfates.

If the concentration of these impurities is not satisfactorilycontrolled it is impossible to secure a deposit of manganese having therequired purity and the maintenance of satisfactory electricalconditions in the cell is seriously impaired. The seriousness of theproblem is increased by the fact that even though the impurities may bereduced to a concentration of very low magnitude, nevertheless theconcentration of these impurities in the electrolyte tends tocontinuously increase because of the fact that the body of theelectrolyte is maintained substantially constant and there iscontinuously added thereto catholyte liquid obtained by extracting freshore containing said impurities. In other words, even though thecatholyte liquid entering the catholyte chamber through the line Hcontains very small concentrations of impurities, the continuousaddition of these impurities to the body of electrolyte gradually buildsup higher concentrations unless adequate means are devised for thecontrol thereof.

Among the advantages of manganese obtained by the electrowinning thereofare not only the low cost of manganese produced by this process ascompared with the thermal proces and the fact that it is possible toemploy low grade manganese ores, but also the fact that manganese may beproduced by electrowinning in a high state of purity. In fact the purityof manganese produced by the electrowinning process must be extremelyhigh as this is one of the outstanding advantages of electrolyticmanganese as compared with manganese obtained by other methods.Consequently the control of impurities introduced into the electrolyteis an important matter not only in respect of controlling theimpurities-of the manganese deposit but also in preventing sidereactions introduced by the impurities and maintaining the efficiency ofoperation, and has presented a serious problem. Not only do theseimpurities when less adequately controlled serious ly contaminate thedeposit of manganese, but also seriously interfere with the successfuloperation of the electrolytic process.

The problem is particularly acute in the case of magnesium owing to thedifiiculty of successfully separating it from the manganese. Iron can beremoved satisfactorily to a large extent by precipitating as ferrichydroxide. Cobalt and nickel can be precipitated to a very large extentby means of ammonium sulfide. Since manganese is commonly in the form ofmanganese sulfate, the anolyte liquid contains a considerableconcentration of sulfuric acid and the relative insolubility of calciumsulfate prevents undue contamination of the electrolyte with calcium.Difflculty has, however, been experienced in adequately separatingmagnesium and various attempts have been made to remove magnesium andcontrol the proportion thereof in the electrolyte without success,however, prior to the present invention. For example, it has beenattempted to precipitate magnesium as magnesium ammonium phosphate. Thisattempt has been a failure, however, because manganese is simultaneouslyprecipitated.

If the magnesium content be allowed to build up without control complexsulfates of magnesium with manganese and ammonium ion are formed in theanolyte. These complex sulfates precipitate out in the anolyte and tendto cause clogging of the diaphragm, the anolyte outlet and the anolytemanifolds. In addition these complex sulfates cause a loss of manganeseand ammonium sulfate which is obviously undesirable. With clogging ofthe diaphragm increased voltages are necessary to overcome the increasedelectrical resistance of the diaphragm with subsequent increase in thevoltage distribution in the cell and a disturbance of the necessarycathode potential for the deposit of manganese metal from the catholyte.If the magnesium content were allowed to build up in an uncontrolledamount in the catholyte with increased alkalinity of the catholyte,basic salts of magnesium tend to be formed which in turn may bemechanically carried to the cathode and deposited with the manganesemetal thereby contaminating it, or else tend to'clog the diaphragm andthe overflow pipes connecting the catholyte with the anolyte chamber. Itis apparent that the deposition of contaminating influences on thecathode will also disturb electrical conditions in the cell.

It has been discovered that it is not necessary to completely removemagnesium because a certain small concentration not exceeding about 4grams of magnesium per liter doesnot cause the formation of troublesomeprecipitates and therefore can be tolerated.

It is an object of the present invention to control the proportion ofmagnesium in an electrolyte containing magnesium and manganese.

It is a further object to limit the proportion of magnesium to an amountnot exceeding about 4 grams of magnesium per liter.

It is a further object to efficiently and economically separatemagnesium from manganese in an electrolyte containing salts of thesemetals.

It has been discovered that soluble fluorides preferentially precipitatemagnesium in the presence of manganese.

It, has been further discovered that this precipitation is best carriedout at a pH between about 1 and 3, preferably, however, 1.5 to 2.75.These discoveries and their application have enabled the above and otherobjects to be accomplished.

In view of the discovery that the separation of magnesium from manganeseis best accomplished within a limited pH range, and further in view ofthe fact that it is possible to control the pH of the anolyte liquidbetween the desired limits, the invention further comprises as one ofthe objects thereof the removal of anolyte liquid from the anolytechamber and treatment thereof with soluble fluorides at the abovementioned pH ranges to remove magnesium prior to employing the anolyteliquid as the means of extracting fresh ore.

The invention as to its principles and scope will be defined in theclaims ultimately appended hereto. An illustrative embodiment will beset forth in the following description taken in conjunction with Fig. 1and Fig. 2, the latter showing diagrammatically a flow-sheet of aprocess carried out in accordance with the present invention. I

Referring specifically to Fig. 2 which shows this flow-sheet, anolyteliquid containing manganese, as manganese sulfate, at a concentration ofabout 5 to 15 grams of manganese per liter and ammonium sulfate at aconcentration of, forexample, about to grams per liter is withdrawn fromthe anolyte chamber 2 to the tank l2 where its pH concentration isadjusted to a, preferred range of about 1.5 to 2.75. This may beaccomplished by the addition of acid or alkali as necessary, and ifalkali is required this is most advantageously supplied by adding therequired proportion of catholyte liquid which may be supplied throughthe line I! provided with valve I5A (see Fig. 1). Instead of catholyteliquid, ore may in some cases be employed. In the tank l2 the anolyteliquid having its pH adjusted as stated is then treated with a. suitablesoluble fluoride. This may be a fluoride of an alkali metal or ammoniumor a double fluoride, as for example, ammonium hydrogen fluorideNHQI'IF'Q. Hydrogen fluoride may be used but it is diflicult to handle.Instead'of using soluble fluorides as such, the reaction product of aninsoluble fluoride and sulfuric acid may be used, e. g., the reactionproduct 01 a slurry-of calcium fluoride and sulfuric acid. Theproportion of soluble fluoride necessary to remove a given quantity ofmagnesium is based upon the equation Mg+ +2F=MgF2 Ina. typical case theconcentration of magnesiumwithdrawn from the anolyte chamber 2 is about4 grams per liter and by treatment with a soluble fluoride is reduced toabout 2 grams per liter, so that when the fluoride treated anolyte isused to extract fresh ore the magnesium addedby this extraction-will notresult in a concentration of magnesium greater than about 4 grams perliter in the resulting liquid fed from the purifying system it to thecatholyte chamber through the line M. It is to be understood that themagnesium increase or build-up per cycle may be so small thatintermittent rather than continuous removal of magnesium from theanolyte is needed. In the typical case referred to the anolyte in thetank i2 is treated with ammonium hydrogen fluoride to the extent ofgrams per liter. This provides the theoretical amount of fluoride ion toremove 22 grams per liter of magnesium and about per cent excess. Theanolyte liquid is then filtered in the filter G5. The filtrate is thenpassed to the leaching tank: 3 and used therein to extract conditionedmanganese ore. The ratio of ore to the filtrate is controlled so thatthe magnesium content of the extract does not exceed about 4 grams ofmagnesium per liter. The liquid extract, which may have a pH of 2.0 to4.0, is then. treatedfor the removal of iron and arsenic by oxidizingthe iron to the ferric condition and adjusting the pH by addition ofalkali to that value which will cause the precipitation of ferrichydroxide. The preferred pH is 5.6 but may be 5.5 to 7.2. The oxidationmay be brought aboutby blowing with air or by treatment with othersuitable oxidizing agents. The precipitate is filtered and the filtrateis then treated with an agent to remove nickel and cobalt as for exampleammonium sulfide. The liquid at this stage is then treated with suchsoluble sulfide, as for example ammonium sulfide, and filtered to removethe precipitate of cobalt and nickel sulfides. The filtrate is thendelivered as catholyte to the catholyte chamber 3.

Instead of causing magnesium separation in the anolyte prior to its useto extract ore in the digester, such separation may occur at any pointin the system where the pH may advantageously be controlled at about 1to 3, preferably 1.5 to 2.75 as for example in the digester or afterdigestion (of fresh ore) and prior to iron precipitation. In such a caseanolyte having a pH range of about 0.5 to 1.5 is used to extract freshore and the pH of the extract is then increased to a range of about 1.5to 2.75.

We claim l. The process of controlling the magnesium content ofelectrolytes used in the electrowin- 'ning of manganese containingmanganese in the form of manganese sulfate together with magnesiumsulfate as an impurity, which comprises treating said electrolyte with asoluble fluoride, partially precipitating magnesium as magnesiumfluoride and separating it from the electrolyte, leaving the remainderof the magnesium in solution to an extent not greater than about 4 gramsper liter and removing excess fluoride.

2. The process of controlling the magnesium content of an electrolyteused in the electrowinning of manganese, said electrolyte .containingmanganese sulfate, ammonium sulfate, sulfuric acid and magnesiumsulfate, which comprises treating said electrolyte at a pH of about 1 to3 with a soluble fluoride, partially precipitating magnesium asmagnesium fluoride and separating it from the electrolyte, leaving theremainder of the magnesium in solution to an extent not greater thanabout 5 grams per liter and. removing excess fluoride.

3. The process of controlling the magnesium content of an electrolyteused in the electrowinning of manganese, said electrolyte containingmanganese sulfate, ammonium sulfate, sulfuric acid and magnesiumsulfate, which comprises treating said electrolyte at a pH of about 1.5to 2.75 with a proportion of soluble fluoride insuflicient toprecipitate all the magnesium, precipitating a portion of the magnesiumfluoride and separating it from the electrolyte, leaving the remainderof the magnesium in solution and removing excess fluoride.

4. The process of controlling the magnesium content of the catholyte andanolyte in the electrowinning of manganese which comprises withdrawinganolyte from the anolyte chamber, said anolyte containing manganesesulfate, am-

monium sulfate, magnesium sulfate and sulfuric oride, leaving theremainder of the magnesium in solution to the extent of about 2 grams ofmagnesium per liter, filtering the precipitate and extractingconditioned manganese ore with the filtrate.

5. The process of limiting the magnesium content of the catholyte andanolyte in the electrowinning of manganese to a value not substantiallyabove about 4 grams of magnesium per liter, which comprises withdrawinganolyte liquor from the anolyte chamber, said anolyte containingmanganese sulfate, ammonium sulfate, magnesium sulfate and sulfuricacid, treating it with a proportion of a soluble fluoride insuficient toprecipitate all of the'magnesium, precipitating a portion of themagnesium and reducing its concentration to a value of about 2 grams perliter, filtering the precipitate, containing magnesium as an impurity,extracting conditioned manganese ore with said filtrate, and controllingthe ratio of ore to said filtrate so that the magnesium content of'theresulting extract is not substantially greater than about 4 grams perliter. Y

6. The process of controlling the magnesium content of the catholyte andanolyte in the electrowinning of manganese which comprises withdrawinganolyte from the anolyte chamber, said anolyte containing manganesesulfate, ammonium sulfate, magnesium sulfate and sulfuric acid, treatingit with a proportion of soluble fluoride at a pH of about 1.5 to 2.75ins'ufllcient to precipitate all of the magnesium, precipitating aportion of the magnesium as magnesium fluoride, leaving the remainder ofthe magnesium in solution to the extent of not more than about 4 gramsof magnesium per liter, filtering the precipitate, extractingconditioned manganese ore with the filtrate, treating said extract toremove therefrom heavy metals other than manganese t content of thecatholyte and anolyte in the tially above 4 grams of magnesium perliter, which comprises withdrawing anolyte liquor from the anolytechamber, said anolyte containing manganese sulfate, ammonium sulfate,magnesium sulfate and sulfuric acid, treating it with a proportion of asoluble fluoride insuficient to precipitate all of the magnesium,precipitating a portion of the magnesium and reducing its concentrationto a Value substantially less than 4 grams per liter, filtering theprecipitate, extracting conditioned manganese ore 'with said filtrate,treating said extract to remove therefrom heavy metals other thanmanganese and to remove excess fluoride, separating said heavy metalsand employing said purified extract as catholyte liquid.

8. The process of controlling the magnesium electrowinning of manganesewhich comprises extracting conditioned manganese ore containingmagnesium as an impurity with said anolyte containing sulfuric acid andhaving a pH of 0.5 to 1.5 and obtaining an extract containing manganesesuifam and magnesium sulfate and having a pH of about 1.5 to 2.75,treating said extract with a proportion of a soluble fluorideinsufficient to precipitate all of the magnesium, precipitating aportion of the magnesium and leaving the remainder in solution to theextent of not more than about 4 grams of magnesium per liter andremoving excess fluoride.

CHARLES L. MAN'I'EIL. WILLIAM L. HAMNIERQUIST.

